The present invention relates to pneumatic radial ply runflat passenger tires and to management of tread lift and vehicle handling during runflat operation.
Various methods have been devised for enabling the safe continued operation of unpressurized or underpressurized vehicle tires with the intent of minimizing further damage to the uninflated tire and without simultaneously compromising vehicle handling over a distance from the place where the tire has lost its pressure to a place desired by the driver, such as a service station, where the tire can be changed. Loss of tire pressure can result from a variety of causes, including puncture by a foreign object such as a nail or other sharp object piercing the pneumatic tire installed on a vehicle.
Pneumatic tires designed for sustained operation under conditions of unpressurization or underpressurization are also called runflat tires, as they are capable of being driven in the uninflated, or what would generally be called xe2x80x9cflatxe2x80x9d, condition. The conventional pneumatic tire collapses upon itself when it is uninflated and is carrying the weight of a vehicle. The tire""s sidewalls buckle outward in the circumferential portion of the tire where the tread contacts the ground, making the tire xe2x80x9cflat.xe2x80x9d
The term xe2x80x9crunflatxe2x80x9d is generally used to describe a tire that is designed in such a way that the tire structure alone has sufficient rigidity and strength to support the vehicle load when the tire is operated in the uninflated condition. The sidewalls and internal surfaces of the tire do not collapse or buckle onto themselves, and the tire does not otherwise contain or use other supporting structures or other devices to prevent the tire from collapsing.
An example of a runflat tire design is described in U.S. Pat. No. 4,111,249, entitled the xe2x80x9cBanded Tire,xe2x80x9d in which a hoop or annular band approximately as wide as the tread is circumferentially deployed beneath the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition.
Numerous methods have been used to achieve workable runflat tire designs. Generally, such tires incorporate sidewall designs that are thicker and/or stiffer, so that the tire""s load can be carried by an uninflated tire with minimum adverse effects upon the tire itself and upon vehicle handling until such reasonable time as the tire can be repaired or replaced. The methods used in sidewall stiffening include the incorporation of circumferentially disposed inserts in the inner peripheral surface of the sidewall portion of the carcass, which is the region in the tire usually having the lowest resistance to deformation under vertical loading. In such runflat tire designs, the sidewalls are thickened in a way that each is approximately uniformly thick in the sidewall region between the bead and the tread shoulder.
The reinforced sidewalls of such tires, when operated in the uninflated condition, experience a net compressive load. However, the outer portions of the reinforced sidewalls are in tension due to bending forces which deflect the sidewalls outward or apart from one another in the regions of the sidewall that are adjacent to the ground-contacting portion of the tread. The inner portions of such reinforced sidewalls, in the region near where the tread contacts the ground, tend to be in compression during runflat operation.
A Goodyear U.S. Pat. No. 5,368,082 (""082) disclosed a low aspect ratio runflat pneumatic radial ply tire which employs special sidewall inserts to improve stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb load in this uninflated tire. This earlier invention, although superior to prior attempts at runflat tire design, still imposed a weight penalty that could be offset by the elimination of a spare tire and the tire jack. However, this weight penalty was even more problematic when the tire designers attempted to build high-aspect-ratio tires for large luxury touring sedans. These taller sidewalled tires, having aspect ratios in the 55% to 65% range or greater, produces correspondingly greater sidewall bending stresses than were encountered in the earlier low-aspect-ratio runflat tires disclosed in the ""082 patent. Thus the sidewalls of high profile tires had to be stiffened to the point of compromising ride characteristics. Luxury vehicle owners generally do not wish to sacrifice ride quality for runflat capability. The design requirements for runflat tire design require that there be no loss in ride quality or handling performance. In the very stiff suspension performance type vehicle, such as sport cars and various sport/utility vehicles, the ability to provide such runflat tires is relatively straightforward compared to providing similar runflat tires for luxury sedans which require softer ride characteristics. Light trucks and sport utility vehicles, although not as sensitive to ride performance, provide a runflat tire market that ranges from accepting a stiffer ride to demanding the softer luxury type ride.
In general, runflat tire design is based on the installation of one or more inserts inside each sidewall flex area. The inserts in each sidewall, in combination with the plies, add rigidity to the sidewalls in the reduction or absence of air pressure during runflat operation. While the high resistance to compression deflection of the inserts provides the necessary resistance to the collapse of the uninflated loaded tire, this method has several drawbacks which include the above mentioned increase in tire weight as well as, during runflat operation, changes in the tread-to-road contact, which adversely affect vehicle handling, especially during high-speed runflat operation.
The changes in the tread-to-road contact during runflat operation derive from the tendency of the thick reinforced sidewalls to transmit bending forces to the portion of the tread that contacts the ground. The result is that the central portion of the tread tends to buckle upwards from the ground. The upward buckle reduces the ground contact in the tread""s central region, resulting in compromised vehicle handling as well as reduced runflat tread life due to severe cyclical flexure, and corresponding heat buildup, of the central portions of the tread, especially during sustained high-speed operation.
Clearly, the goal in runflat tire design is to provide a tire which gives good runflat vehicle handling as well as good service life of the tire during runflat operation. Since the handling deficit associated with runflat operation derives at least partially upward buckling of the central portion of the tire tread, various methods have been introduced to minimize such buckling or xe2x80x9ctread lift,xe2x80x9d as it is sometimes called. Such methods include means by which to increase the lateral stiffness of the tread by the employment of additional structures, such compressive-stress-bearing breaker reinforcements, tensile-stress-bearing ply reinforcements, and wedge inserts located between the breakers and plies. (The latter wedge inserts give mechanical advantage to the respective tension-bearing and compression-bearing structures). The main disadvantage of such tread-stiffening structural additions is greater tire weight, in addition to the increased weight of the reinforced sidewalls.
Another aspect of the tire design that effects runflat operation relates to the tread contour. Treads designed specifically for the passenger vehicles traditionally have been designed such that the cross-sectional, or translateral, contour of the tread has been defined according to multiple radii of curvature which are joined in such a way that adjacent circular curves meet one another tangentially, i.e., without discontinuity in the curvature of the tread across its lateral dimension. Furthermore, the radius of curvature defining the cross-sectional contour of the central portions of the tread tends to be much longer than the radii of curvature that define the cross-sectional contour of the tread in the shoulder regions.
An alternative tread contour is one in which the translateral contour is defined by circular curves that do not join tangentially, i.e., the tread contour is defined by adjacent circular curves which intersect nontangentially. An example of such a tread design is taught in patent application Ser. No. PCT/US98/00717 entitled PASSENGER TIRE WITH IMPROVED TREAD CONTOUR WITH DECOUPLING GROOVES having a common assignee with the present invention and is incorporated in its entirety by reference hereto. The tread contour described in patent application Ser. No. PCT/US98/00717 is one in which nontangential, i.e., discontinuous, circular curves define the translateral contour of the tread in such a way that the lateral-most portions of the tread shoulders are xe2x80x9cdropped,xe2x80x9d or angled radially inward, such that the central portion of the tread becomes more heavily loaded than the shoulder portions of the tread during both normal inflated operation and during runflat operation. Thus one objective of the tread contour described in patent application Ser. No.PCT/US98/00717 was to achieve an improved tread footprint in tires incorporating at least one carcass ply in which the reinforcing cords are of high-modulus material. High-modulus carcass plies, especially in combination with the insert reinforced sidewall structures that are characteristic of current runflat tire designs, are more readily able to transmit bending forces, or torques, during runflat operation, from the sidewalls to the tread. A result of such transmitted bending forces is, as described above, a tendency of the center portions of the tread to buckle upward, i.e., to undergo the kind of xe2x80x9ctread liftxe2x80x9d during runflat operation which leads to compromised runflat vehicle handling.
More specifically, in the particular case of runflat type tires having thick sidewalls, if the tread shoulder contours are described by tangentially joined adjacent curves having smaller radii of curvature in the regions of the tread shoulders, bending forces tend more readily to be transmitted from the sidewalls to the tread during conditions of low to zero air pressure in the tire. One result of this upward buckling of the tread during runflat conditions is the potential for loss of vehicle handling in straight-run operation and, especially, in cornering; oversteering is particularly problematic when the uninflated runflat tire is located on a rear axle.
Additional difficulties associated with runflat tire designs having cross-sectional tread contours defined by mutually tangential circular curves is shorter operational life of the tread in the runflat condition, due to the cycling of severe upward bending stresses in the tread center. Yet another result associated with runflat operation is the tendency for the shoulders of the tire to come into contact with the road, causing yet further difficulties in handling in straight line operation and especially in cornering.
The disadvantages of tread contours defined by tangentially intersecting adjacent circular curves described in the paragraph above in relation to runflat tires apply generally to all tires that have cross-sectional tread contours which are defined by tangentially adjacent circular curves.
However, the development of tires having high-modulus carcass plies, in particular plies that are reinforced with inextensible metal cords, leads to treads that tend not to flatten in a way that optimally distributes the loading on the ground; more specifically, a high-modulus carcass having low radii of curvature tread contour in the shoulder region tends to cause the tire""s load to be disproportionately borne by the shoulders and sides of the tread, with the central portions of the tread being under little or no load under runflat conditions. This leads to poor footprint pressure distribution in straight driving and to low footprint contact surface in cornering.
It is an object of the present invention to provide a runflat radial tire as defined in one or more of the appended claims and, as such, having the capability of being constructed to accomplish one or more of the following subsidiary objects.
One object of the present invention is to provide a runflat radial tire whose tread is isolated from sidewall bending forces that tend to lift the central portions of the tread away from the ground during runflat operation.
Another object of the present invention is to provide a runflat radial tire having a radially outermost sidewall that includes a rib which makes contact with the road during runflat operation, thereby increasing the effective width of the tread and maintaining good high-speed vehicle handling in the runflat mode.
Another object of the present invention is to provide a runflat radial tire having a cross-sectional tread contour that facilitates the uniform flattening of the entire tread width upon the road during runflat operation.
Yet another object of the present invention is to provide a runflat radial tire incorporating decoupling grooves axially inward of each lateral-most tread rib and in each crown shoulder so as to minimize the transmission of bending forces that originate in the sidewalls during runflat operation and which tend to lift the central portions of the tread out of contact with the road.
Still another object of the present invention is to apply the inventive concept to a variety of alternative carcass constructions.
The present invention relates to a pneumatic radial ply runflat tire having a tread, a carcass comprising a radial ply structure having at least one radial ply, a belt structure located between the tread and the radial ply structure, two sidewalls reinforced by one or more inserts, and a tread contour of which the laterally disposed tread ribs are defined by circular curves having large radii of curvature. The outermost ply, or the single ply, is reinforced with inextensible metal cords. The sidewalls each have a rib near the radially outermost regions. The circular curves that define the cross-sectional contour of the central portions of the tread and the laterally disposed tread rib intersect nontangentially. A circumferentially disposed decoupling groove underlies each respective nontangential locus of points of nontangential intersection of the circular curves that define the cross-sectional contour of the tread. The circular curve defining the contour of each radially outward-most sidewall rib intersects nontantentially with the circular curve that defines the contour of each laterally disposed tread rib. A second set of decoupling grooves is disposed such that one groove is located circumferentially in each shoulder region where the contour-defining curves intersect nontangentially between each radially disposed sidewall rib and the adjacent laterally disposed tread rib. The lateral-most decoupling grooves between the laterally disposed tread rib and the sidewall rib are circumferential and continuous, or they are circumferential and non-continuous. The decoupling grooves between the laterally disposed tread rib and the central portions of the tread are circumferential and straight in design, or they are have a zig-zagged pattern.
The preferred embodiment of the invention is a pneumatic runflat radial tire having a low-aspect-ratio (in the range of about 30% to about 60%) design. This embodiment has potential for runflat use in high-performance sports type vehicles or light trucks. The inventive features of this low-aspect-ratio, radial ply runflat pneumatic tire provide for runflat operation in which runflat tread lift is minimized, and the tread footprint is widened during runflat operation.